Method for storing a subcooled liquid

ABSTRACT

An enclosed cryogenic storage tank having an insulated bottom, a vertical insulated wall projecting upwardly from the bottom, an insulated roof projecting to the wall and completely covering the space surrounded by the wall, and an insulated deck inside and extending across the tank at a fixed location between the tank bottom and tank roof. A process of removing a subcooled liquefied gas from the space beneath an insulated deck in an insulated cryogenic storage tank, feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, returning the subcooled liquefied gas to the tank below the deck, and feeding the cooled stream of liquefied gas to the tank above the deck. Alternatively, a stream of the subcooled liquefied gas can be mixed with the steam of warm liquefied gas and the combined stream then fed to the tank above the insulated deck.

United States Patent 1 Laverman [111 3,812,683 I451 May 28, 1974 METHODFOR STORING A SUBCOOLED LIQUID [75] Inventor: Royce Jay Laverman, SouthHolland, 111.

[22] Filed: May 15, 1972 [2l] Appl. No.: 253,110

Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. CaposselaAttorney, Agent, or Firm-Merriam, M arshall, Shapiro & Klose [57]ABSTRACT An enclosed cryogenic storage tank having an insulated bottom,a vertical insulated wall projecting upwardly from the bottom, aninsulated roof projecting to the wall and completely covering the spacesurrounded by the wall, and an insulated deck inside and extendingacross the tank at a fixed location between the tank bottom and tankroof.

A process of removing a subcooled liquefied gas from the space beneathan insulated deck in an insulated cryogenic storage tank, feeding theremoved subcooled liquefied gas in heat exchange with a stream of warmliquefied gas of the same composition as the subcooled liquefied gas tocool the warm stream, returning the subcooled liquefied gas to the tankbelow the deck, and feeding the cooled stream of liquefied gas to thetank above the deck. Alternatively, a stream of the subcooled liquefiedgas can be mixed with the steam of warm liquefied gas and the combinedstream then fed to the tank above the insulated deck.

8 Clainis, 3brawingFigures 1 METHOD FOR STORING A SUBCOOLED LIQUID Thisinvention relates to processes and apparatus for storing liquids. Moreparticularly, this invention is concerned with improvements in liquidstorage tanks and the use of such tanks in processes of cooling warmliquids.

Storage tanks of various sizes are used to store liquids. The tankshave'various shapes including spherical, elliptical and cylindrical. Themost common shape of ,tank is one which has a flat bottom, verticalcircular cylindrical wall and a conical or domed roof. Such tanks areusedto store materials which are liquid at ambient temperatures andpressures, as well as gases which have been refrigerated to liquid form.To retard heat leak, storage tanks for liquefied gases are fullyinsulated.

Because of the low boiling point of many liquefied gases, it isadvisable to store them in insulated tanks at a temperature which givesa vapor pressure of atmospheric pressure or slightly thereabove. Thisnecessitates cooling the liquefied gases to. very low temperatures. Ifthe low boiling liquefied gases were to be stored at higher pressures,the strength of the tank, and capital cost,- would need to be greatlyincreased. It is generally less expensive to cool' the liquefied gas toa low temperature suitable for storage at atmospheric pressure thantobuild atank adequate to store the product at a higher pressure.

In addition to storing liquefied gases at atmospheric pressure and aboiling point temperature consistent therewith. it is sometimesdesirableto subcool a liquefied gas below its boiling point and to storeit at such temperature in a tank to provide stored refrigeration. Thus,a subcooled heel of liquefied gas can be provided in the tank to storerefrigeration which could later be used to minimize vapor, generationwhen a warm stream of liquefied gas is fed to the tank for storage. Asubcooled liquefied gas, however, will cause a vacuum in'the tank vaporspace because of the subcooled temperature and if means are not providedto prevent this, the tank may. be crushed by atmospheric pressure.Increasing the strength of the tank to withstand the vacuum is not adesirable solution because of the increased cost. An alternative meansis accordingly needed to provide for suitable storage of a subcooledliquefied gas, and particularly storage of subcooled liquefied gas inthe same tank to be used for storage of a larger quantity of theliquefied gas at its boiling point under atmospheric pressure.

According to one aspect of the invention there is pro vided an insulatedliquid storage tank enclosing a volume and having an insulated deckinside the tank extending across an area of the tank and positioned andfixed to be below the full level, and abovethe empty level. of the tank.The deck'can be fixed stationary in one position or location,'or it canbe arranged to be vertically movable within a certain vertical distancebetween the full and empty levels of the tank with the said movementbeing limited by suitable stops, chains or other restraining means. Aquantity of subcooled liquefied gas can be stored in the tank in thespace below the deck to cool a stream of warm liquefied gas fed to thetank interior space above the deck. The insulated deck prevents a vacuumfrom developing in the tank above the deck when a subcooledliquefiedjgas is stored below the deck. As a result, the tank need-notbe able to withstand a pressure differential caused by a reduction ininternal pressure. The deck also serves to separate warmer liquid placedabove the deck from colder liquid below it.

An opening is advisably located in the deck to permit flow of liquidand/or vapor from above the deck to below the deck and, in somecircumstances, from below the deck to above it. Furthermore, the deck isadapted to be totally submerged in liquefied gas in the tank. It istherefore essential that it be constructed to withstand hydrostaticpressures applied to it. In addi-' tion, the deck advisablyis built sothat it has no or very little buoyancy when submerged so as to avoid theapplication of significant upward forces on the tank wall. The deckhowever is advisably made slightly buoyant when it is to be displaceablewithin predetermined limits in the tank. An advantage of having the deckdisplaceable to a limited extent is that it permits an increase in thevolume or space below the deck for storing subcooled liquid.

The shape of the deck is notnarrowly critical. For most purposes howeveran essentially flat horizontal deck is suitable, and particularly one ofsubstantially uniform thickness. Regardless of the shape of the deck,

it can be supported by the tank wall, or by posts or columns extendingfrom the deck to the tank surface there below, or by suspension membersextending from the tank roof to the deck, or any combination of suchsystems. The deck nevertheless is advisably made rigid although for somepurposes it can be flexible.

The tank is generally provided with one or more conduits communicatingfrom the tank space below the deck to outside the tank. It is alsoadvisably provided with a conduit communicating from the tank vaporspace above the deck to outside the tank.

According .to a further aspect of the invention, the tank can be used ina process of cooling warm liquefied gas for storage in the tank. Thus,subcooled liquefied gas from the space beneath the insulated deck can beremoved and placed in heat exchange with a warm stream of liquefied gasof the same general composition. The warm stream is thereby cooled,after which it can be fed to the space in the tank above the deck forstorage at,'or about at, atmospheric pressure. The subcooled liquefiedgas can then be returned to the tank space below the deck. In analternative process, the subcooled liquefied gas removedfrom the tankspace below the deck can be mixed with the warm stream of liquefied gasand the combined stream then fed to the tank space above the deck forstorage at atmospheric pressure. Such processes are especially usefulfor cooling a warm stream of a cryogenic liquefied gas, such as naturalgas which is being unloaded after transportation by ship orbarge. Thesubcooled liquefied gas provides ready refrigeration to cool the warmstream.

The invention will be described further in conjunction with the attacheddrawings in which:

FIG. 1 is a vertical cross section'of a storage tank having an insulateddeck;

FIG. 2 is a schematic representation of means to cool a warm stream ofliquefied gas after ship transportation using the storage tank of FIG.1; and I FIG. 3 is a schematic representation of another means to cool awarm stream of liquefied gas after ship transportation using the storagetank of FIG. 1.

So far as is practical the same identifying numbers will be used for thesame or similar parts which appear in the various drawings. v

With reference to FIG. 1, tank has an outer shell comprising a flatmetal outer bottom 11, a vertical circular cylindrical outer wall 12 anda domed metal roof 13'. An inner shell comprising flat metal innerbottom '14 and vertical cylindrical circular metal inner wall 15 ispositioned insideof the outer shell with insulation 16 between outerbottom 11 and inner bottom 14 and insulation 17 between outer wall 12and inner wall 15. Insulated ceiling 20 has metal sheet 18, suspended byrods2l from roof 13, which supports insulation 19 thereon. Tank 10 isintended to store a liquefied gas at, orslightly above, atmosphericpressure.

Insulated deck is horizontally positioned in tank 10 above inner bottom14 in fixed position. The deck 25 thus divides the tank interior volumeinto a space 26 below the deck and a space 27 above the deck.

Columns 28, vertically positioned and spaced apart from one another,support deck 25 above inner bottom 14. The columns can be made of anysuitable material but advisably are made of metal; Guy rods' 29 serve tohold the columns vertical.v 1

Insulated deck 25 has a metal bottom 31,.metal top 32 and insulation33ther'ebetween. The edge of the deck extends to the surface of inner wall15 and, .if advisable, can be joined thereto. Holeor opening 34 ex-'tends through deck 25 so there can be flow of liquid back and forthbetween the spaces above and below the deck. v

Conduit 35 communicates with conduit 37 through valve 36, and conduit 37communicates with space 26 below deck 25 for delivering liquefied gasthereto.

I Conduit 38 also communicates withspace 26 below deck 25 and it can beused for removing liquefied gas from the tank. Conduit 38 communicateswith conduit 40 through control valve 39.

- Conduit extends through insulated ceiling 20 and domed roof l3. Vaporgenerated during filling of the tank, or by heat leak during storage,can be removed by conduit 45 to prevent the internal pressure fromrising morev than slightly above atmospheric pressure. Conduit 45 alsopermits air or some other'gas to flow into the tank during emptyingt'o'avoid the creation of a vacuum in the tank high enoughto cause it tocollapse fromatmospheric pressure."

Subcooled liquefied gas can be stored in space 26 below insulated deck25. Because 'of the insulated deck, the subcooled liquid 'will not causethe vapor pressureof the portion'of liquid stored in the space above thedeck to be lowered below atmospheric pressure. Tank 10 accordingly neednot be constructed as a vacuum vessel with the increased costs thiswould involve. Tank 10 thus can be constructedto store a liquefied gasat atmospheric pressure.

Representative ways in which the tank of FIG. I can be used areillustrated schematically by FIGS. 2 and 3, it being understood thatsuch conduits as are required are incorporated into tank 10 to make-itsuitable for the intended purpose.

With reference to FIG. 2, ship arrives at a destination carryingliquefied natural gas at approximately 254 F. and 16.7 psia. The warmliquefied natural gas is removed from ship 50 by pump15l which deliversit to conduit 52 which feeds it to .pump'53. From pump 53 the liquefiednatural gas isfed to conduit 55 which.

feeds it through heat exchanger tubes 56 in heat exchanger 57. Theliquefied natural gas is fed, after being cooled toapproximately 258 F.at l5.2 psia from tubes56 to conduit 58 which feeds the liquefiednatural gas to space 27 in tank 10 above insulated deck 25.

Subcooled liquefied natural gas having an initial temperature ofapproximately -285 F. and a vapor pressure of about 3.4 psia iswithdrawn by conduit 59 from space 26 below deck 25 by means of pump 60and is fed to conduit 61 which delivers it to heat exchange tubes 62inheat exchanger 57. The Subcooled natural gas provides refrigeration,as it passes through the heat exchanger, to cool the warm liquefiednatural gas passing through tubes 56. The subcooled liquefied naturalgas at approximately -260 F. is fed from tubes 62 to conduit 63 whichreturns it to space 26 below deck 25.

The system shown schematically in FIG. 3 also uti- I lizes the noveltank of FIG. 1. With reference to FIG. 3, Subcooled natural gas havingan initial temperature of approximately -285 F. .and a vapor pressure of3.4

psia IS removed from space 26 by conduit and pump 71 which delivers thestream of Subcooled liquefied natural gas to conduit 72. The warm streamof liquefied natural gas at approximately 254 F. and l6.7 psia removedfrom ship 50 (as described with reference to FIG. 2) is fed by pump 53to conduit 73. The stream of Subcooled liquefied natural gas is fed byconduit 72 to conduit 73 where the two streams are intermixed to form asingle stream alt-approximately ,258 F. and 15.2 psia. Conduit 73 feedsthe combined single stream to space 27 above deck 25 for storage atatmospheric pressure.

' As liquefiednatural gas is removed by conduit 70 from below deck 25,an equal volume of liquefied natural gas flows from the space 27 abovedeck 25 through hole 34into the space 26 below the deck.

It is highly advisable in'practicing the invention to always maintainsome liquid at a temperature consistent with storage at atmosphericpressure above deck 25 when space 26 contains a subcooled liquefied gasso that creation of a vacuum by reduction of the vapor pressure in space27 is avoided. Desirably a volume of liquefied gas a few feet thick ismaintained on deck 25 at all times so that the vapor pressure in space27 will always be maintained at some small positive value as contrastedwith a vacuum.

The subject invention .has several advantages over the use of anunrestricted floating roof. One advantage is that less insulation isrequired because the temperature differential between space '27 andspace 26 is not great compared to the temperature differential betweenthe temperature of liquefied gas below an insulated floating roof andthe temperature of vapor above it. Furthermore, whenan insulatedfloating roof is used, the liquefied gas to be stored must be feddirectly into the mass of subcooled liquefied gas being used as tank ofthis invention, the subcooled liquefied gas is maintained generallyseparate from the product being stored and thus at a lower temperaturemore suitable for providing refrigeration to a warm stream. Anotheradvantage is the lack of moving parts when a fixed roofed is used.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed isi l. The process which comprises:

providing an insulated storage tank having an insulated deck thereinwith subcooled liquefied gas below the deck,

removing the subcooled liquefied gas from the space beneath theinsulated deck in the insulated cryogenic storage tank, s'aid deck beingpositioned and fixed to be between the full level and empty level of thetank,

feeding the removed subcooled liquefied gas in heat exchangewith astream of warm liquefied gas of the same composition as the subcooledliquefied gas to cool the warm stream, 3 1 returning the subcooledliquefiedgas to the tan below the deck, and I feeding the cooled streamof liquefied gas to thetank above the deck. 2. The process of claim 1,in which the liquefied is liquefied natural gas.,

3. The process of claim 1, in which the pressure in the tank above thedeck is at an internal pressure of generally atmospheric pressure.

4. The process which comprises: providing an insulated storage tankhaving an insulated deck therein with subcooled liquefied gas below thedeck,

removing a subcooled liquefied gas stream from the space beneath theinsulated deck in the insulated cryogenic storage tank, said deck beingpositioned and fixed to be between the full level and empty level of thetank,

combining the removed subcooled liquefied gas gas stream with a streamof warm liquefied gas of the I 5. The process of claim 4, in which thedeck has an opening through which liquefied gas can flow from above tobelow the deck.

6. The process of claim 4, in which the pressure in the tank is abovethe deck at an internal pressure of generally atmospheric pressure.

7. The process which comprises: providing an insulated storage tankhaving an insulated deck therein with subcooled liquified gas below thedeck,

removing the subcooled liquefied gas from the space beneath theinsulated deck in the insulated cryogenic storage tank, said deck beingpositioned and fixed to be between the full level and empty level of thetank and permitting liquid flow from the space on either side of thedeck to the space on the other side,

feeding the removed subcooled liquefied gas in heat exchange with astream of warm liquefied gas of tioning an insulated deck within saidtank, said deck having an aperture extending from both sides through thedeck surrounding the deck with liquefied gas including a subcooledliquefied gas volume beneath said deck, i removing a portion of thesubcooled liquefied gas from beneath the deck for heat exchange purposesand causing a flow of liquefied gas from above the deck through theaperture'thereby replacing the removed subcooled volume of liquefied gasbelow the deck, maintaining the insulated deck surrounded with liquefiedg'as, v I feeding the removed'subcooled liquefied gas in heat exchangewith a stream of warm liquefied gas of the same composition as thesubcooled liquefied gas to cool the warm stream, and I returning thevolume'of liquefied gas, removed from belowthe deck, and I the cooledstream of liquefied gas to the tank above the deck. I

1. The process which comprises: providing an insulated storage tankhaving an insulated deck therein with subcooled liquefied gas below thedeck, removing the subcooled liquefied gas from the space beneath theinsulated deck in the insulated cryogenic storage tank, said deck beingpositioned and fixed to be between the full level and empty level of thetank, feeding the removed subcooled liquefied gas in heat exchange witha stream of warm liquefied gas of the same composition as the subcooledliquefied gas to cool the warm stream, returning the subcooled liquefiedgas to the tank below the deck, and feeding the cooled stream ofliquefied gas to the tank above the deck.
 2. The process of claim 1, inwhich the liquefied gas is liquefied natural gas.
 3. The process ofclaim 1, in which the pressure in the tank above the deck is at aninternal pressure of generally atmospheric pressure.
 4. The processwhich comprises: providing an insulated storage tank having an insulateddeck therein with subcooled liquefied gas below the deck, removing asubcooled liquefied gas stream from the space beneath the insulated deckin the insulated cryogenic storage tank, said deck being positioned andfixed to be between the full level and empty level of the tank,combining the removed subcooled liquefied gas stream with a stream ofwarm liquefied gas of the same composition as the subcooled liquefiedgas, and feeding the combined stream of liquefied gas into the tankabove the deck.
 5. The process of claim 4, in which the deck has anopening through which liquefied gas can flow from above to below thedeck.
 6. The process of claim 4, in which the pressure in the tank isabove the deck at an internal pressure of generally atmosphericpressure.
 7. The process which comprises: providing an insulated storagetank having an insulated deck therein with subcooled liquified gas belowthe deck, removing the subcooled liquefied gas from the space beneaththe insulated deck in the insulated cryogenic storage tank, said deckbeing positioned and fixed to be between the full level and empty levelof the tank and permitting liquid flow from the space on either side ofthe deck to the space on the other side, feeding the removed subcooledliquefied gas in heat exchange with a stream of warm liquefied gas ofthe same composition as the subcooled liquefied gas to cool the warmstream, returning the subcooled liquefied gas to the tank below thedeck, and feeding the cooled stream of liquefied gas to the tank abovethe deck.
 8. The process which comprises: providing an insulated liquidstorage tank and positioning an insulated deck within said tank, saiddeck having an aperture extending from both sides through the deck,surrounding the deck with liquefied gas including a subcooled liquefiedgas volume beneath said deck, removing a portion of the subcooledliquefied gas from beneath the deck for heat exchange purposes andcausing a flow of liquefied gas from above the deck through the aperturethereby replacing the removed subcooled volume of liquefied gas belowthe deck, maintaining the insulated deck surrounded with liquefied gas,feeding the removed subcooled liquefied gas in heat exchange with astream of warm liquefied gas of the same composition as the subcooledliquefied gas to cool the warm stream, and returning the volume ofliquefied gas, removed from below the deck, and the cooled stream ofliquefied gas to the tank above the deck.